Since the beginning of the month, more than 550 people have been killed in Iraq. Experts are left struggling to explain the escalation of violence.

The latest attacks by insurgents have been largely in the centre and north of the country. There have been attacks in different parts of Baghdad, in Tikrit about 175km (110 miles) to the north, and, further north, in the town of Hawija, near Kirkuk.

There has also been fierce fighting between US forces and suspected Islamic militants in what is often called Iraq's "wild west", near the Syrian border.

A big US operation is under way against a suspected network of the radical Jordanian Islamist, Abu Musab al-Zarqawi.

The violence has shattered the lull which followed the Iraqi election at the end of January.

April saw a sharp increase in attacks, especially suicide bombings, and this month there has so far been no let-up.

Experts offer various explanations as to who is behind the escalation of violence and why.

US officials now see the foreign "jihadi" fighters as their most important, and most ruthless, enemies - even though, numerically, these fighters are outnumbered by Iraqi insurgents.

The insurgency seems to have shrunk as its tactics have become more vicious, according to senior US officials quoted this week in the Washington Post.

The US still appears to know little about the insurgents

The same officials believe former loyalists of the Saddam Hussein regime are reassessing their strategy in the light of the election. Some of them seem ready to abandon violence and enter the political game.

This helps explain why US forces are targeting the "jihadis" with a large-scale assault in the west of the country. They are convinced foreign fighters are continuing to cross from Syria into this lawless desert region.

Many of them join the network run by Zarqawi.

US forces have the twin aim of clearing out an area that has become a haven for Islamists and smugglers and, if possible, killing or capturing Zarqawi.

Political vacuum

As to why there has been such a surge of attacks by insurgents, the most widely-shared view links it to Iraq's messy political evolution.

Three months of haggling over the creation of a new government created a political vacuum which insurgent groups have sought to exploit.

Now that a new government has been sworn in, these groups have an interest in trying to undermine its credibility. But that is only a theory.

What is striking, more than two years after the war which toppled Saddam Hussein, is how little the Americans appear to know about their enemy.

There are thought to be dozens of insurgent groups, with differing agendas. They sometimes act autonomously, sometimes in loose co-operation.

The stark truth may be that no one can know for sure whether there is a pattern to the insurgency, or why the violence ebbs and flows.